Magnetic Random Access Memories (MRAMs) are a promising alternative to conventional dynamic semiconductor memories. MRAMs are nonvolatile memories, which, in contrast to conventional dynamic semiconductor memories, do not need a refresh process for information retention. MRAM memory cells are substantially formed of two magnetic layers with a nonmagnetic separating layer arranged in between. MRAMs are resistant to radiation, so that information retention is ensured even when radiation is incident.
An MRAM memory cell is based on ferromagnetic storage with the aid of the tunneling magnetoresistance (TMR) effect or the giant magnetoresistance (GMR) effect. In accordance with its conventional construction, a layer stack having a soft-magnetic layer or storage layer, a tunnel oxide layer and a hard-magnetic layer or reference layer is arranged at the crossover point between bit and word lines arranged in crossed fashion. Magnetization of the reference layer is predefined, while the magnetization of the storage layer is adjustable by sending corresponding currents in different directions through the word line and the bit line. By these currents, the magnetization of the storage layer can be set parallel or antiparallel with respect to the magnetization of the reference layer. In the case of a parallel magnetization of storage layer and reference layer, the electrical resistance in the stack direction of the layer stack (i.e., from top to bottom or vice versa) is less than in the case of an antiparallel magnetization of storage layer and reference layer. This electrical resistance dependent on the different magnetization directions of the two layers can be evaluated as logic state “0” or “1”.
The magnetization of the storage layer that is parallel or antiparallel with respect to the reference layer is enabled by a magnetic anisotropy of the storage layer, which defines a magnetic preferred direction. The expression “preferred direction” is conventionally used, although “preferred axis” would be more correct since both directions along the axis are equally preferred. Despite this, the expression “preferred direction” is used here.
Such a magnetic anisotropy can be provided by shape anisotropy. Thus, in the case of a magnetic layer shaped in elongate fashion in terms of its spatial form, the magnetic preferred direction corresponds to the geometrical longitudinal direction of the magnetic layer. Due to the physical conformity to law that the leakage field energy is to be as low as possible, in energetic terms, the magnetization is directed collinearly with respect to the preferred direction of the anisotropy. By applying an external magnetic field, the magnetization of the storage layer can be switched back and forth between the two energetically preferred positions, if the activation energy required to overcome the energetically unfavorable intermediate positions is made available by the external magnetic field. In practice, such a shape anisotropy of memory cells is realized, for example, by magnetic layers that are elliptically shaped in terms of their spatial form.
In the case of rotationally symmetrical magnetic layers, by contrast, the magnetic anisotropy is obtained as an intrinsic material property since an “in-plane” shape anisotropy cannot be realized. The cause of intrinsic anisotropy is under discussion, but electron diffraction data at amorphous layer materials permit the conclusion that anisotropic orientation of atomic pair axes in the direction of the magnetic field is a possible cause of the intrinsic anisotropy.
In conventional, known MRAM memory cells, magnetization of the storage layer is set parallel or antiparallel with respect to the magnetization of the reference layer since this makes it possible to obtain a maximum signal swing with regard to the change in resistance ΔR/R of the layer stack during magnetization reversal of the magnetization of the storage layer relative to the magnetization of the reference layer.
However, with a circular-disk-shaped configuration of the memory cell and in the weak intrinsic anisotropy of the storage layer, it is not possible to ensure that the magnetization of the storage layer is oriented collinearly with respect to the preferred direction. In general, a single cycle of the magnetization reversal of the storage layer establishes a remnant magnetization of the storage layer, in which case the magnetization is directed non-collinearly with respect to the preferred direction of the intrinsic anisotropy of the storage layer.
This phenomenon will now be explained in more detail with reference to FIGS. 1A-1C. FIGS. 1A-1C show a storage layer (FIGS. 1A and 1B) and a reference layer (FIG. 1C) of a conventional MRAM memory cell. To facilitate reference, FIGS. 1A-1C are provided with an X, Y axis system in which the X axis points toward the right in the horizontal direction, while the Y axis points upward in the vertical direction. The reference layer 1, a layer exhibiting hard-magnetic behavior with a circular-disc shape, is magnetized along the X axis, which is represented symbolically by the arrows 2. The storage layer 3, a layer exhibiting soft-magnetic behavior with a circular-disk shape, has a strong intrinsic anisotropy with a preferred direction along the double arrow. The preferred direction of the intrinsic anisotropy is accordingly oriented along the X axis or −X axis. In the interior of the storage layer 3, the magnetization symbolized by the arrows 4 substantially follows the course of the preferred direction of the intrinsic anisotropy. Furthermore, the magnetization 4 in the interior of the storage layer 3 is substantially oriented parallel to the magnetization 2 of the reference layer 1.
With regard to the storage layer 3 of FIG. 1A, the illustration shows a state of the storage layer after the application and switching-off an substantially homogeneous, external magnetic field 7 directed through 90° in the counterclockwise direction relative to the X axis (remnant state). Through the action of the external magnetic field 7, the magnetization 4 in the interior of the memory cell 3 is oriented parallel to the field direction thereof, but after the magnetic field 7 has been switched off, the magnetic field it reverts to a direction substantially parallel to the preferred direction of the intrinsic anisotropy. For the magnetization 5, 6 in the edge regions of the memory cell 3, by contrast, it is energetically advantageous with regard to avoiding high leakage field energies if they remain in a direction parallel to the field direction of the external magnetic field 7, even after the latter has been switched off. Although only a gradual transition of the magnetization 5, 6 of the edge regions to the magnetization 4 in the interior of the storage layer is made possible on account of the magnetic exchange interaction, the magnetization 4 in the interior of the memory cell 3, on account of the strong intrinsic anisotropy, assumes an orientation parallel to the preferred direction thereof. During magnetization reversal, the magnetization in the interior of the storage layer 3 is a collinear, i.e., parallel or antiparallel, orientation with respect to the preferred direction. As a result magnetization 4 in the interior of the storage layer 3 and the magnetization 2 of the reference layer 1 are oriented collinearly with respect to one another and a maximum signal swing with regard to the change in resistance ΔR/R of the layer stack is possible.
The behavior of a circular-disk-shaped storage layer having a weak intrinsic anisotropy differs from this, and this behavior will be explained with reference to FIGS. 2A and 2B, left-hand illustration. In order to facilitate reference, FIGS. 2A-2C are provided with an X, Y axis system in the same way as FIGS. 1A-1C.
FIGS. 2A and 2B show a storage layer 10 exhibiting soft-magnetic behavior with a circular-disk shape. The storage layer 10 has a weak intrinsic anisotropy with a preferred direction along the double arrow. As the storage layer 3 was in FIGS. 1A and 1B, the storage layer 10 is in a state after the application and switching-off of an substantially homogeneous, external magnetic field 7 directed through 90° in the counterclockwise direction relative to the X axis (remnant state). In this case, after the magnetic field 7 has been switched off, the magnetization 12, 13 in the edge regions of the storage layer 10 avoid high leakage field energies by remaining in a direction parallel to the field direction of the previously applied magnetic field. However, the magnetization in the interior of the storage layer 10 cannot attain an orientation parallel to the preferred direction due to the weak intrinsic anisotropy of storage layer. In other words, the influence of the magnetization 12, 13 in the edge regions of the storage layer 10, due to the magnetic exchange interaction, prevents a collinearity between the magnetization 111 in the interior of the storage layer 10 and the preferred direction of the intrinsic anisotropy. Consequently, a remnant magnetization 12 is established in the interior of the storage layer 10. The magnetization is oriented at an angle α in the counterclockwise direction relative to the X axis.
The explanations that the expression “strong intrinsic anisotropy” in the sense of the present invention is intended to denote those storage layers of an MRAM memory cell in which, during magnetization reversal of the storage layer, an substantially collinear orientation between the remnant magnetization in the interior of the storage layer and the preferred direction of the intrinsic anisotropy always occurs, while in contrast thereto, in the case of storage layers having a “weak intrinsic anisotropy,” a remnant magnetization which is oriented non-collinearly with respect to the preferred direction occurs in the interior of the storage layer. A weak intrinsic anisotropy is typically accompanied by an anisotropy field strength of less than approximately 1 kA/m (approximately 12.6 Oe).
If the remnant magnetization of the storage layer is not directed parallel or antiparallel with respect to the magnetization direction of the reference layer, this has the very disadvantageous consequence that it is not possible to obtain the maximum signal swing with regard to the change in resistance ΔR/R of the layer stack during magnetization reversal of the magnetization of the storage layer relative to the magnetization of the reference layer. As shown by a computational consideration when a remnant magnetization occurs in the storage layer, it is possible to obtain a signal swing weighted with cos α with regard to the change in resistance ΔR/R. Therefore, such storage layers cannot be used, or can be used only in a very restricted manner, for application in MRAM memory cells.
An MRAM memory cell having a circular-disk-shaped geometry of the layers of the layer stack and only weak intrinsic magnetic anisotropy of the storage layer to avoid a reduced signal swing with regard to the change in resistance ΔR/R of the layer stack during magnetization reversal of the magnetization of the storage layer relative to the magnetization of the reference layer on account of a remnant magnetization of the storage layer occurring is desirable.